Intrinsic and Extrinsic Parameters of Foods That Affect Microbial Growth
49
was metabolic. It appears from this study that a low a w forces S. cerevisiae to divert a greater proportion of its metabolic activity to glycerol production accompanied by an increase in the amount of
glucose consumed during growth. In a later study, it was noted that up to 95% of the external osmotic
pressure exerted on S. cerevisiae, Z. rouxii, and Debaryomyces hansenii may be counterbalanced by
an increase in glycerol.
43 Z. rouxii accumulates more glycerol under stress, whereas ribitol remains
constant.
It is known that the growth of at least some cells may occur in high numbers at reduced a w values,
while certain extracellular products are not produced. For example, reduced a w results in the cessation
of enterotoxin B production by S. aureus even though high numbers of cells are produced at the same
time.
50,51 In the case of Neurospora crassa, a low a w resulted in nonlethal alterations of permeability
of the cell membrane, leading to a loss of several essential molecules.
12 Similar results were observed
with electrolytes or nonelectrolytes.
Overall, the effect of a lowered a w on the nutrition of microorganisms appears to be of a general
nature where cell requirements that must be mediated through an aqueous milieu are progressively
shut off. In addition to the effect on nutrients, a lowered a w undoubtedly has adverse effects on the
functioning of the cell membrane, which must be kept in a fluid state. The drying of internal parts of
cells would be expected to occur upon placing cells in a medium of lowered a w to a point where the
equilibrium of water between cells and substrate occurs. Although the mechanisms are not entirely
clear, all microbial cells may require the same effective internal a w . Those that can grow under extreme
conditions of a low a w apparently do so by virtue of their ability to concentrate salts, polyols, and
amino acids (and possibly other types of compounds) to internal levels sufficient not only to prevent
the cells from losing water, but that it may allow the cell to extract water from the water-depressed
external environment. For more information, see references 49, 51.
Oxidation–Reduction Potential
It has been known for decades that microorganisms display varying degrees of sensitivity to the
oxidation–reduction potential (O/R, Eh) of their growth medium.
23 The O/R potential of a substrate
may be defined generally as the ease with which the substrate loses or gains electrons. When an element
or compound loses electrons, the substrate is oxidized, whereas a substrate that gains electrons becomes
reduced:
Cu ←−
−→ Cu + e
oxidation
reduction
Oxidation may also be achieved by the addition of oxygen, as illustrated in the following reaction:
2Cu + O 2 → 2CuO
Therefore, a substance that readily gives up electrons is a good reducing agent, and one that readily
takes up electrons is a good oxidizing agent. When electrons are transferred from one compound to
another, a potential difference is created between the two compounds. This difference may be measured
by use of an appropriate instrument, and expressed as millivolts (mV). The more highly oxidized a
substance, the more positive will be its electrical potential; the more highly reduced a substance, the
more negative will be its electrical potential. When the concentration of oxidant and reductant is equal,
a zero electrical potential exists. The O/R potential of a system is expressed by the symbol Eh. Aerobic
microorganisms require positive Eh values (oxidized) for growth, whereas anaerobes require negative
49
was metabolic. It appears from this study that a low a w forces S. cerevisiae to divert a greater proportion of its metabolic activity to glycerol production accompanied by an increase in the amount of
glucose consumed during growth. In a later study, it was noted that up to 95% of the external osmotic
pressure exerted on S. cerevisiae, Z. rouxii, and Debaryomyces hansenii may be counterbalanced by
an increase in glycerol.
43 Z. rouxii accumulates more glycerol under stress, whereas ribitol remains
constant.
It is known that the growth of at least some cells may occur in high numbers at reduced a w values,
while certain extracellular products are not produced. For example, reduced a w results in the cessation
of enterotoxin B production by S. aureus even though high numbers of cells are produced at the same
time.
50,51 In the case of Neurospora crassa, a low a w resulted in nonlethal alterations of permeability
of the cell membrane, leading to a loss of several essential molecules.
12 Similar results were observed
with electrolytes or nonelectrolytes.
Overall, the effect of a lowered a w on the nutrition of microorganisms appears to be of a general
nature where cell requirements that must be mediated through an aqueous milieu are progressively
shut off. In addition to the effect on nutrients, a lowered a w undoubtedly has adverse effects on the
functioning of the cell membrane, which must be kept in a fluid state. The drying of internal parts of
cells would be expected to occur upon placing cells in a medium of lowered a w to a point where the
equilibrium of water between cells and substrate occurs. Although the mechanisms are not entirely
clear, all microbial cells may require the same effective internal a w . Those that can grow under extreme
conditions of a low a w apparently do so by virtue of their ability to concentrate salts, polyols, and
amino acids (and possibly other types of compounds) to internal levels sufficient not only to prevent
the cells from losing water, but that it may allow the cell to extract water from the water-depressed
external environment. For more information, see references 49, 51.
Oxidation–Reduction Potential
It has been known for decades that microorganisms display varying degrees of sensitivity to the
oxidation–reduction potential (O/R, Eh) of their growth medium.
23 The O/R potential of a substrate
may be defined generally as the ease with which the substrate loses or gains electrons. When an element
or compound loses electrons, the substrate is oxidized, whereas a substrate that gains electrons becomes
reduced:
Cu ←−
−→ Cu + e
oxidation
reduction
Oxidation may also be achieved by the addition of oxygen, as illustrated in the following reaction:
2Cu + O 2 → 2CuO
Therefore, a substance that readily gives up electrons is a good reducing agent, and one that readily
takes up electrons is a good oxidizing agent. When electrons are transferred from one compound to
another, a potential difference is created between the two compounds. This difference may be measured
by use of an appropriate instrument, and expressed as millivolts (mV). The more highly oxidized a
substance, the more positive will be its electrical potential; the more highly reduced a substance, the
more negative will be its electrical potential. When the concentration of oxidant and reductant is equal,
a zero electrical potential exists. The O/R potential of a system is expressed by the symbol Eh. Aerobic
microorganisms require positive Eh values (oxidized) for growth, whereas anaerobes require negative